Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal-oxide-semiconductor (CMOS) image sensors, has continued to advance at great pace. For example, the demands of higher resolution and lower power consumption have encouraged the further miniaturization and integration of these image sensors.
Conventional CMOS image sensors typically include an array of pixels, where each pixel includes a photodiode that transforms incident light into an electrical charge. Each individual pixel has an output that, for a fixed exposure time, eventually saturates with increasing light intensity. Saturation of the photodiodes can produce unwanted image smearing due to an effect known as blooming, where excess charge spreads into neighboring pixels.
Full-well-capacity is the measure of the amount of charge which can be accumulated in the photodiode before such saturation occurs. As the pixel-size of CMOS image sensors become smaller, the active area of the photodiode is also reduced. The smaller photodiode area leads to a smaller full-well-capacity. The reduced full-well-capacity typically means a lower dynamic range and lower signal-to-noise ratio.
Recently, CMOS image sensors have been fabricated that include both large-area and small-area pixels so as to increase the dynamic range of the image sensor. That is, large-area pixels, with corresponding large photodiode areas, may be used for low-light conditions, whereas small-area pixels, with corresponding small photodiode areas, may be used for non-low-light conditions. However, conventional fabrication techniques may not necessarily translate when fabricating an image sensor that includes both large-area and small-area pixels. For example, the traditional methods of forming microlenses over an array of pixels may need to be updated so as allow for the formation of dual-sized microlenses corresponding with the small-area and large-area pixels.